1. Field of the Invention
Embodiments of the present disclosure relate to security alarm systems and associated methods for protecting residences, businesses and other premises. More particularly, the present disclosure relates to ground fault detection within the circuitry of a security alarm system and identifying the type of ground fault condition once detected.
2. Discussion of Related Art
Security or alarm systems are installed in premises to detect hazardous or potentially hazardous conditions. A security system generally includes a plurality of detectors/sensors, one or more keypads and a control panel which contains much of the system electronics. The control panel may include a communication interface for remote monitoring and two-way communication over telephone or wireless communication paths with a remote monitoring station. Each of the detectors may communicate with the control panel to provide its current status as well as notification of an alarm condition. Examples of possible alarm conditions may include unauthorized entry or the unexpected presence of a person who may be an intruder, fire, smoke, toxic gas, high/low temperature conditions (e.g., freezing), flooding, power failure, etc. In other words, an alarm condition may represent a detectable condition that might lead to personal hazard or property damage. Audible and/or visible alarm notification devices such as sirens, lights, etc., may also be utilized to notify occupants of the existence of an alarm condition. The control panel may be located in a utility room, basement, etc., and may communicate with the detectors and notification devices over wired or wireless signal paths. A keypad, which may also communicate with the control panel over a wired or wireless connection, may be used to arm/disarm the system as well as providing a means to display various system messages via a status display screen.
Maintaining the integrity of electrical connections between and among the various detectors/sensors, notification devices, keypads and control panel is of great importance to ensure that these devices are functioning properly. Ground fault conditions may present a breakdown in system integrity. A ground fault condition is generated when a minimum leakage current is flowing to earth ground; in particular if any wire connected to the alarm system is shorted or heavily coupled to earth ground. In the case of an alarm system, if any wire connected to the alarm system should touch or create a leakage to earth-ground, a ground fault should be indicated. Thus, it is desirable to detect, identify and notify of a ground fault condition in the control panel that may compromise the integrity of a system. If left unaddressed, ground fault conditions may render the security system inoperable.
One type of ground fault condition is a non-isolated panel connection to earth ground. A non-isolated panel connection may include certain of the wired control panel connections to other system components such as, for example, the detectors, keypads and notification devices. One common approach to detecting such a ground fault condition involves injecting a steady DC current to earth ground and measuring the voltage drop between earth ground and circuit ground. The voltage drop is proportional to the ratio of earth ground to circuit ground resistance. This may typically involve a current generator to supply the DC current that may be coupled to the higher voltage of the circuit to drive the ground resistance. The voltage drop between earth ground and circuit ground is then evaluated to detect whether a ground fault has occurred. For example, a detected resistance that is smaller than a specific expected value may indicate an unspecified ground fault condition in the system. The above solution, however, is not immune to DC level shifting. Generally, DC level shifting may occur when interfacing different types of circuits to each other, such as when interfacing circuits operating at one particular voltage level to circuits operating at another voltage level. The above solution cannot categorize, identify, or distinguish between a capacitive coupling induced ground fault condition and a resistance induced ground fault condition. In addition, a capacitive coupling type of ground fault condition may increase the probability of a control panel malfunction due to possible component coupling to a noise source. It is with respect to these and other considerations that the present improvements have been needed.